The present invention relates to a multi-stage solvent extraction process of a metal value such as rare earth elements or, more particularly, to an improvement in a multi-stage solvent extraction process of a metal value from an aqueous acidic solution containing the metal value into a water-immiscible organic extractant solution.
The process of solvent extraction or liquid-liquid extraction as a kind of the technologies for material separation is widely employed in the industries of non-metallic and metallic materials. For example, a metal value in a trace concentration can be efficiently recovered or collected by the solvent extraction method and a metallic element can be purified or separated from impurities or accompanying metallic elements, which can hardly be separated by a conventional chemical method, by solvent extraction. The basic procedure of solvent extraction method of a metal value is conducted by bringing an aqueous solution containing the metal value in the form of a water-soluble salt into contact with a water-immiscible organic solvent or an organic solution containing an extractant agent dissolved therein. The extractant compound is specific to the particular metallic element so that the metal value is selectively transferred from the aqueous phase to the organic phase.
The process of solvent extraction can be classified into two types depending on the nature of the extractant agent. In the solvent extraction process of the first type, the extractant agent is a neutral compound, such as tributyl phosphate (TBP) and quaternary ammonium compounds, with which the metallic element forms a complex compound to be extracted into the organic phase. Taking a tervalent metallic element denoted by M, such as a rare earth element, in the form as a nitrate of the metallic element and TBP as the extractant compound, the process of solvent extraction of this type can be expressed by the equation: EQU [M(NO.sub.3).sub.3 ].sub.A +3[TBP].sub.O =[(TBP.sub.3). M(NO.sub.3).sub.3 ].sub.O,
in which the subscript A or O at the right-hand side of the respective square brackets means that the species inside of the square brackets exists in the aqueous phase or in the organic phase, respectively.
In the solvent extraction process of the second type, which is a so-called ion-exchange solvent extraction process, the extractant agent is exemplified by phosphorus-containing organic acids, carboxylic acids, and .beta.-diketone compounds with which the metallic ions are combined with release of an acid or hydrogen ions H.sup.+. Denoting a phosphorus-containing organic acid soluble in an organic solvent by HP, the solvent extraction process of a tervalent metallic ions M.sup.3+ can be expressed by the following equation: EQU [M.sup.3+ ].sub.A +3[HP].sub.O =[M(P).sub.3 ].sub.O +3[H.sup.+ ].sub.A,
in which the subscripts A and O at the right-hand side of the square brackets each have the same meaning as explained above. The phosphorus-containing acidic extractant compound can be in the form of a dimer or trimer. Although the metal value in the above explanation is assumed to be in the form of a simple cation, the same principle of mechanism is applicable to complex ions formed between the metallic ions and anionic complex-forming ions.
In the solvent extraction process of the latter type, which is more efficient than the solvent extraction process of the former type, as is understood from the above given equation and in contrast to the solvent extraction process of the former type, the hydrogen ion concentration in the aqueous phase is increased as the extraction of the metallic ions proceeds leading to establishment of a state of extraction equilibrium between the aqueous phase and organic phase so that the process of extraction cannot proceed any further. A conventional method for accomplish further proceeding of the solvent extraction process is to decrease the hydrogen ion concentration in the aqueous phase by the addition of an alkaline compound, such as alkali metal hydroxides and ammonia water, to the aqueous phase thus to neutralize a part of the free acid therein. Needless to say, neutralization of a free acid, e.g., hydrochloric and nitric acids, with an alkaline compound produces a salt such as sodium chloride and ammonium nitrate. Accordingly, the solvent extraction process of the latter type, when practiced in a large scale, has disadvantages due to the large consumption of the alkaline compounds as a neutralizing agent and by-production of a large amount of salts to be disposed without causing any environmental pollution resulting in a great increase in the production costs.